An elongated rotor for a steam turbine, for example, is constructed by assembling axially-divided rotor components. A flange portion of each rotor component has coupling holes for coupling to an adjacent rotor component. A horizontal boring machine having a main shaft which is translatable in orthogonal three axes (X-, Y- and Z-axes) is generally used to produce the precision coupling holes in each rotor component.
In order to couple rotor components directly, the coupling holes are required to be machined with machining accuracy on the order of 30 μm in terms of the cylindricity, position, diameter, etc. of the holes; otherwise a need arises to perform hole machining at coupling portions of the rotor components upon assembling of the rotor components, leading to an increased cost. A calibration method has therefore been developed which not only corrects a translational error in the direction of each of the linear axes (X-, Y- and Z-axes), but also measures and corrects a three-dimensional volumetric error. In particular, a calibration system “Laser Tracer” manufactured by Etalon, Germany, achieves measurement accuracy on the order of submicrons. According to such a technology, in addition to a translational error and a rotational error in the direction of each of the linear axes (X-, Y- and Z-axes), an error in the squareness of the overlapping three axes can also be measured. This makes it possible to measure and correct a three-dimensional spatial error with high accuracy. However, though the conventional method can three-dimensionally correct a volumetric error at the front-end point of a tool, it cannot correct an inclination of a main shaft which may occur upon feeding of the main shaft during deep-hole boring as performed by means of a horizontal boring machine.